1. Field of the Invention
The present invention relates to a liquid ejection apparatus, a liquid ejection method and an image forming apparatus, and more particularly, to a liquid ejection apparatus which ejects liquid through nozzles connected to pressure chambers by applying pressure change to the liquid inside the pressure chambers by using the displacement of piezoelectric elements.
2. Description of the Related Art
There is known a recording head used in an inkjet recording apparatus that ejects droplets of ink through nozzles connected to pressure chambers by applying pressure change to the ink inside the pressure chambers by using the displacement of piezoelectric elements. In a recording head based on this piezoelectric method, and in particular when using an ink of high viscosity, a phenomenon referred to as “trailing” is liable to occur in which the ink droplet ejected from a nozzle forms a column shape and trails during its flight. This can give rise to minute ink droplets referred to as “satellite droplets” accompanying the main ink droplet, which degrade the image quality.
In order to resolve problems of this kind, as one means of preventing excessive elongation of the liquid column, for example, it has been considered that the surface tension of the liquid is raised to increase the speed of growth of the initial necking, in such a manner that the ligament separates off at an early stage from a main droplet portion having acquired momentum; however, in actual practice, the surface tensions of non-metallic liquids are limited to approximately 70 mN/m, which is the surface tension of water. Moreover, in practical terms, if the surface tension is excessively raised, then the actuator power required to break the free surface (also referred to as the “meniscus”) of the liquid inside the nozzle increases, and further problems occur in relation to the permeability of the liquid into the recording medium after landing of the liquid droplets on the recording medium. Furthermore, it may be thought that conditions can be optimized to ensure that the liquid column is ejected without excessive elongation, by reducing the ejection pressure to a limit value at which the main droplet portion breaks off before the free surface of the liquid is pulled back into the nozzle due to the surface tension acting on the liquid column; however in this case, the ejection speed is reduced and in the case of fine liquid droplets in particular, it is difficult to deposit the droplets onto the recording medium due to air resistance.
On the other hand, Japanese Patent Application Publication No. 2004-066531 discloses that the free surface of charged ink is deformed into a thread shape by means of a first electrode disposed in a position opposing the nozzle, and the front end portion of the free surface of the ink that has been deformed into the thread shape is severed by means of a second electrode disposed between the nozzle and the first electrode.
Japanese Patent Application Publication No. 2004-066531 assumes the use of a recording apparatus of a continuous inkjet (CIJ) type, and supposes that the free surface of the ink is formed into the thread shape; however, in a recording apparatus of a drop on demand (DOD) type, which is typical in a piezoelectric system, the principal focus is on preventing the occurrence of satellite droplets by restricting elongation of the liquid column, and it is difficult to apply the composition described in Japanese Patent Application Publication No. 2004-066531 to the recording head that uses the piezoelectric method. This is because in the piezoelectric type of recording head, in order to prevent satellite droplets, it is necessary to suppress the thread shape (the trail of the liquid column), as much as possible, but if the electrode composition according to Japanese Patent Application Publication No. 2004-066531 is combined with the piezoelectric type of recording head, then the pulling electrode in the vicinity of the target continues to pull on the liquid column and it is impossible to avoid elongation of the liquid column.
Moreover, in Japanese Patent Application Publication No. 2004-066531, the actions of driving (electrostatic pulling action), charging and severing are each carried out using different electric fields, and therefore significant cross-talk between the electric fields is expected. In particular, in a multi-nozzle composition, there are concerns regarding the effects of electric field cross-talk between the nozzles. Furthermore, if high voltages are applied to the respective electrodes, then there is a risk of electrical breakdown between the electrodes.